1. Field of the Invention
The present invention may generally relate to a relay system based on an orthogonal frequency division multiple access (OFDMA) scheme. More particularly, the present invention may relate to an apparatus and a method for removing self-interference (SI) in the relay system by using a relay reference signal (R-RS).
2. Background of the Related Art
International Mobile Telecommunication—Advanced (IMT-Advanced) has been developed to provide multimedia services based on an internet protocol (IP) with data rates of 1 Gbps for fixed or low mobility nodes and 100 Mbps for high mobility nodes. Relay, spectrum aggregation, uplink Multiple-Input and Multiple-Out (MIMO) and interference cancellation between cells are the main technologies which have been studied for Long Term Evolution—Advanced (LTE-Advanced).
Wireless repeaters are used to remove shadow areas of cells. Further, the wireless repeaters are set up at boundaries of the cells to extend cell coverage effectively and improve throughput.
Currently, frequency converting repeaters are mainly used as wireless repeaters, and Interference Cancellation System (ICS) repeaters have been developed recently and used to improve the use efficiency of the frequency, which cannot be provided with frequency converting repeaters.
Self Interference (SI) is the interference incurred on a receiving antenna by a transmitting antenna when the signals of the same band are simultaneously transmitted and received by the transmitting antenna and the receiving antenna. SI appears when the same frequency band is used between user equipment and the repeater and between the repeater and a base station. If the repeater transmits signals to the user equipment through a transmitting antenna thereof, the signals are also received at a receiving antenna thereof. Thus, SI occurs when the repeater receives signals from the base station. SI occurs not only at the downlink but also at the uplink.
Frequency converting repeaters adopt an out-band scheme, in which a center frequency of the frequency band used for the backhaul link, between the base station and the repeater, differs from that of the frequency band used for the access link, between the repeater and the user equipment. The ICS repeater adopts the in-band scheme which uses the same center frequencies. Specifically, the frequency converting repeater (the out-band repeater) adopts the scheme in which the frequency converting repeater receives downlink (or uplink) signals from the base station (or the user equipment) and converts the center frequency to remove the effect of the SI when the frequency converting repeater relays the signals to the user equipment (or to the base station). The ICS repeater (in-band repeater) adopts the scheme in which the ICS repeater receives downlink (or uplink) signals from the base station (or the user equipment) and converts the center frequency to remove the effect of the SI when the frequency converting repeater relays the signals to the user equipment (or to the base station). The ICS repeater (the in-band repeater) receives the downlink (or uplink) signals from the base station (or the user equipment). The scheme of eliminating wireless return signals from transmitted (or relayed) signals by a digital signal process is used in the ICS repeater to remove the SI on the receiving antenna which receives the relayed signals before the ICS repeater relays the signals to the user equipment (or to the base station) through the transmitting antenna.
The frequency converting repeater needs an additional frequency band (out band) for the delay. Thus, it uses frequency wastefully and degrades the use efficiency of the wireless frequency. Further, noise on the receiving antenna of the repeater is also amplified and transmitted, and the quality of the received signals deteriorates.
In the case of the ICS repeater, it is difficult to adopt the MIMO scheme, and the efficient improvement of the throughput cannot be anticipated. In detail, the auto-correlation between the retransmitted signals and the receiving signals is used to cancel the SI incurred on the receiving antenna of the repeater which receives the signals transmitted (relayed) by the transmitting antenna. However, exact correlation cannot be obtained easily when desired signals or noise components are large. Thus, the performance of the ICS repeater degrades. Further, if the SI is not removed completely, the signal may be subjected to an oscillation phenomenon.
For the 3rd Generation Partnership Project (3GPP), the standard of using relays has been adopted since the relays are more efficient than wireless repeaters. A time division scheme, in which sections of the transmittance and the receipt are separated in time, has been considered to avoid SI in relays being used in 3GPP. SI occurs when the frequencies of the transmittance and the receipt of the relay are in the same band. The so-called “In-band half-Duplex scheme” is the scheme of using the same frequency band and separating the transmitting/receiving sections in time. Compared with the wireless repeater, the relay has an advantage of the performance improvement by the signal processing through upper layers such as the physical layer, the media access control layer (MAC layer) and so on. Specifically, in the case of downlink (or uplink), an in-band half-duplex relay receives signals from the base station (or the user equipment) at a predetermined time and at a predetermined frequency. Errors in the received signals are corrected through a digital signal process, and the retransmitting to the user equipment (or the base station) is performed after modulating the signals in the form of transmittance. The relay does not transmit signals to the user equipment (or the base station) while receiving the signals from the base station (or the user equipment). Like this, SI can be avoided by separating the sections of the transmitting/receiving in time.
The in-band half-duplex relay can contribute to improve the performance compared with the wireless repeater. However, the latency increases greatly on the system due to the digital signal process required to separate the sections of the transmitting/receiving in time. Further, there is a problem of lowering frequency efficiency due to the use of the source by the time division.